The invention relates to a device for muscle stimulation.
Muscle-driven heart assist systems (for example cardio myoplasty, aorta myoplasty, skeletal muscle ventricle, bio-mechanical hearts) are nowadays already employed in clinical settings, for example experimentally as assist or replacement therapy of heart transplantation and treatment of a terminal heart insufficiency. These muscle assist systems can operate both in parallel and in series with the diseased heart. The systems are intended to relieve (reduce the cardiac wall tension, relieve pressure, relieve volume, relieve post-stress) and also to assist the circulation, i.e., increase the average pressure of the arterial blood pressure and/or to increase the pump volume. Independent of the configuration of the heart assist system, a muscle pacemaker is required for deliberate stimulation of a muscle contraction, which transmits to the muscle to be assisted an electrical stimulation pattern in synchronism with the heart beat via stimulation electrodes. A stimulation pattern consists of a sum of individual pulses which can be characterized by their stimulation voltage, the pulse width and the spacing between two pulses. By a meaningful combination of several individual pulses to a group of individual pulses with a subsequent pause, a stimulation burst is created which can be used cyclically contract and relax the muscle tissue. In addition to the aforedescribed parameters, the number of stimulation pulses per burst and the frequency with which a stimulation burst is applied can be used to describe a stimulation pattern. An additional parameter is the placement of the stimulation burst within the heart cycle, which can be defined by a delay time.
Experiments on large animals have shown that a continuous and frequent application of stimulation burst causes a fiber transformation of the stimulated muscle tissue, with the generated muscle fibers being substantially free of fatigue, but weak and slow. It has been observed that the muscle fiber cross-sections significantly decrease when stimulation bursts are applied continuously and frequently. The muscle tissue, which is here mainly represented by the type-I muscle fibers, is barely suitable to perform the pumping benefiting the circulation. However, experimental tests have shown that before the stimulated muscle fibers are transformed into type-I muscle fibers, they are transformed into an intermediate, already fatigue-free form, which still includes strong type-IIa muscle fibers. This quick and strong muscle tissue, which is dominated by type-IIa muscle fibers, is only preserved if the number of applied stimulation pulses per time interval remains below an upper limit value. Stimulation above this upper limit causes the type-IIa muscle fibers to transformation into type-I muscle fibers, accompanied by a loss in muscle strength and quickness.
Conventional muscle pacemaker systems are capable of supplying predefined or computed stimulation bursts synchronously with and triggered by the heart rhythm, wherein the ratio of muscle contraction to heart contraction is adjustable. This ratio can be predefined as a function of the heart rate. For example, if a high heart rate persists over an extended period of time, then the same high number of stimulation pulses is supplied by the muscle pacemaker. The utilization of the muscular heart assist system is then exceedingly high, so that most of the muscle fibers are transformed into weak and slow type-I muscle fibers. Muscular-driven heart assist systems then lose their effectiveness, so that the diseased heart can no longer be effectively relieved. The support of the circulatory system deteriorates with increased utilization, so that the heart rate of the patient may increase further to compensate this effect. This can cause the supported muscle to completely degenerate.
DE 101 52 741 A1 discloses a heart therapy device, in particular an implantable defibrillator, heart pacemaker or combined heart pacemaker-defibrillator, with an evaluation and control unit for evaluating a measured heart rate. The evaluation and control unit includes a memory with three ranges for storing a first, a second, and a third range of values of comparative heart rates. The adjacent ranges are organized by increasing values of the heart rate. The evaluation and control unit further includes a heart rate discriminator for associating a measured heart rate with the first, second or third range of values, and a stability evaluation unit for evaluating the stability or constancy of the heart rate over a predetermined time interval, when a heart rate in the second range of values is detected. When a heart rate in the second range of values is detected, one of two different therapy control signals is supplied, depending on the stability of the heart rate. The stability criterion is used to distinguish between “rapid” tachycardia (“flutter”) and fibrillations with a-still-relatively low frequency. U.S. Pat. No. 4,406,287 also discloses a pacemaker. This device is intended to terminate tachycardia, wherein different pulse numbers/pulse rates can be employed. If tachycardia is not terminated after applying a first combination of a certain number of pulses and a certain pulse rate, then a second phase is applied with a changed combination of pulse number and pulse rate. The last successful combination of pulse number and pulse rate is then stored as a starting value for terminating the next tachycardia. With this approach, the probability for terminating a tachycardia can be enhanced.
U.S. published patent application 2003/0083703 A1 discloses a method and a device for providing an anti-tachycardia pulse pattern. This device is also capable of determining if a pulse pattern has terminated a tachycardia and if a change in the pulse pattern is indicated.